The half-closed slot for preventing the slipping-out of windings in the armature of the rotary machine is usually formed by providing beforehand a projecting portion in the vicinity of a slot portion on the slot open side of the armature and by bending the projecting portion inside the slot by pressing or the like, as is disclosed, for instance, in U.S. Pat. No. 3,586,893 patented on June 22, 1971 entitled "Armature Core" and Japanese Patent Laid-Open No. 52-9805 (1977) published on Jan. 25, 1977 entitled "Manufacturing Method of Armature of Small-scale rotary machine".
However, the above-stated prior art has disadvantages, as will be described below.
On the occasion when the projecting portion is bent inside the slot by pressing or the like, the fore end portion of this bent projecting portion bends down to cause damage to an insulating coat on the surface of a winding held in the slot, thus tending to cause dielectric breakdown. Moreover, as in the armature of Japanese Patent Laid-Open No. 52-9805 (1977) of the above-mentioned prior part, when the projecting portion is grooved on the lateral side thereof and bent inside the slot, the mechanical strength of the aforesaid projecting portion is deteriorated by a groove provided on the lateral side of the projecting portion for bending, and consequently said portion is sometimes cut off in the course of high-speed rotation. Furthermore, when the half-closed slot is formed by pressing a semicircular projection in the armature core of U.S. Pat. No. 3,586,893 mentioned above, a large load pressure is needed, which requires employment of a large-sized apparatus, and also this art involves a drawback that irregularity is easily formed on the outer peripheral surface of the armature when the half-closed slots are formed by pressuring the semicircular projections, so that the uniformity of magnetic reluctance of the armature tends to be deteriorated.